1. Field of the Invention
The present invention relates to biaxially oriented rigid thermoplastic structures comprising a layer of vinyl chloride polymer and a coating layer of polymethyl methacrylate containing products absorbing ultraviolet radiation.
2. Background of the Related Art
It is known to manufacture, starting with thermoplastic resins such as vinyl chloride polymer (PVC, and the like), finished objects exhibiting improved mechanical characteristics by orienting the macromolecules in one direction or in two directions at right angles. A major increase in rigidity and in impact strength is observed, proportional to the orientation produced.
It is also known, especially from French Patent FR-A-1,571,391, to improve the light- and weather resistance of finished objects made of PVC by coextrusion of a coating layer of a methyl methacrylate homo- or copolymer. In this way, the surface of the finished objects is modified and the good characteristics of polymethyl methacrylate are imparted thereto, insofar as its high gloss, its high weather resistance and its excellent optical characteristics are concerned.
It is also known, especially from German Patent Application DE-A-2,536,461, to produce impact-resistant multilayer composites comprising at least two thermoplastic materials of different impact strength by drawing the composite biaxially at a temperature above the softening point of the materials in question, but at which at least the material which has the lower impact strength is in the thermoelastic state. The material of higher impact strength is, in particular, PVC, while the other one, with good weather resistance, is, in particular, polymethyl methacrylate. The temperature of orientation of the multilayer composite lies in the range 120.degree.-150.degree. C.
The disadvantage of these high orientation temperatures is that the ability of PVC to elongate is low and that the chain relaxation rate is high, with the result that it is not possible to obtain elongations which are sufficient to produce an orientation permitting the final mechanical characteristics to be appreciably improved. In fact, in these conditions, only an elongation of the order of 30% can be obtained by biaxial drawing in the case of PVC, and this is inadequate to produce a sufficient final orientation. The optimum of the temperature of orientation of PVC lies, in fact, in the range of 95.degree.-105.degree. C., close to the glass transition temperature of PVC (85.degree. C.), at which temperature the elongations of PVC is higher than 70 % when drawn biaxially.